1. Field of the Invention
The present invention relates to a device suitable for detecting the abnormal rotation of a workpiece being ground in a non-circular workpiece grinding machine such as cam grinding machine.
2. Discussion of the Related Art
Generally, workpieces having non-circular portions such as cams or the like are ground in numerically controllable grinding apparatuses (i.e., numerical control grinding machines). In such apparatuses, a work spindle for supporting a workpiece is rotatably supported in a work head, and a wheel head rotatably supporting a grinding wheel is movable in a direction intersecting with the work spindle, wherein feed amounts of the wheel head are controlled in a synchronous relation with the rotation of the work spindle. The grinding of each non-circular portion is carried out in the manner of, e.g., varying an override value representing the change rate in the work spindle rotational speed, in dependence on the amount of the non-circular portion ground with the grinding wheel per unit angle rotation of the workpiece or varying the override value for the work spindle rotational speed to make the grinding speed of the non-circular portion constant over the whole circumference thereof.
That is, during a rough grinding, because the grinding resistance built up is smaller at a maximum diameter portion and a base circle portion of each non-circular portion of the workpiece than at other portions, the machining at each of the maximum diameter portion and the base circle portion is performed with the rotational speed of the work spindle increased for a shorter machining time, while the machining at each of other portions than the maximum diameter portion and the base circle portion is performed with the rotational speed of the work spindle decreased because the grinding resistance at each of such other portions is large. That is, there is performed a grinding ability constant control for keeping the grinding amount per unit time constant. Further, during a finish grinding, for enhancement in the machining accuracy, there is performed a so-called grinding speed constant control, wherein the rotational speed of the work spindle is varied in dependence on radii at respective circumferential regions of each non-circular portion so that the grinding speed on each non-circular portion of the workpiece is controlled to be maintained constant.
FIG. 8 shows one example of a known detecting device for detecting abnormal workpiece rotation in the event that a workpiece W with non-circular portions comes not to rotate bodily with a work spindle. In the known device, a proximity switch 200 is provided on a work table 12, mounting thereon a work head (not shown) which supports the workpiece W, to face the circumferential surface of the workpiece W on the side opposite to a grinding wheel G. When a maximum diameter portion (e.g., top portion in the case of a cam) of the workpiece W is not detected by the proximity switch 200, the known device detects that the workpiece W is abnormally rotating relative to the work spindle. Further, as described in JP 5-138422 A, a device for detecting the breakage of a phase pin 3 which serves as a machining phase reference of a workpiece to a work spindle uses a noncontact sensor 4 and detects abnormal workpiece rotation in the event that the workpiece comes not to rotate bodily with the work spindle.
When judging that the maximum diameter portion of the non-circular portion of the workpiece is detected not to be rotating in a synchronous relation with the rotation of the work spindle, these known devices judge the occurrence of an abnormality and discontinue the ongoing machining operation, whereby the non-circular portion being ground of the workpiece or the grinding wheel can be prevented from being damaged.
However, in the aforementioned numerical control grinding machines, the rotational speed of the work spindle being under speed control tends to be increased for higher efficiency. This makes it difficult for the proximity sensor to detect the maximum diameter portion because the rotational speed of the work spindle is high. Therefore, in order to secure a sufficiently wide detection region and hence, a threshold value of a wide range therefor, in other words, in order to secure a sufficient time period (t) for the proximity sensor to detect the maximum diameter portion, it becomes unavoidable to take appropriate measures such as, e.g., by widening the arc of a sensing portion for the proximity sensor which detects the maximum diameter portion of the non-circular portion. This gives rise to a problem that the detection accuracy is deteriorated. This is caused by the configuration that the proximity sensor 200 is provided on the work table 12 mounting the work head, as shown in FIG. 8.
Specifically, in the configuration that the proximity sensor 200 is provided on the side opposite to the grinding wheel G with the workpiece W therebetween, the grinding wheel G is machining a base circle portion on the side opposite to the proximity sensor 200 with the workpiece W therebetween when the proximity sensor 200 is detecting the maximum diameter portion of the non-circular portion. At this time, the work spindle is in a high rotational speed range, so that the proximity sensor 200 is subjected to detecting the maximum diameter portion under the condition of the work spindle rotating at a high speed. Therefore, in order to make the detection by the proximity sensor 200 possible in a high speed rotational range of the work spindle, it is unavoidable to take measures in the course of widening the range of the threshold value which specifies the detection region. However, taking such measures makes it difficult to realize precise detection. Under the foregoing circumstance, there has been desired a device capable of precisely and reliably detecting the abnormal workpiece rotation even under the condition that the rotational speed of the work spindle is varied largely in dependence on the rotational phase of the workpiece while a non-circular portion of the workpiece is being ground.